Various principles and methods are known in the field of electronic and electro-optical distance measurement. One approach consists in emitting pulsed electromagnetic radiation, such as e.g. laser light, to a target to be measured and then receiving an echo from said target as a backscattering object, wherein the distance to the target to be measured is determined on the basis of the time of flight of the pulse. Such pulse time-of-flight measuring elements have gained acceptance in the meantime as standard solutions in many fields.
Various approaches are used for detecting the backscattered pulse.
The so-called threshold value method involves detecting a light pulse if the intensity of the incident radiation exceeds a certain threshold value.
The other approach is based on the sampling of the backscattered pulse. An emitted signal is detected by virtue of the fact that the radiation detected by a detector is sampled, a signal is identified within the sampled region and, finally, the position of said signal is determined. By using a multiplicity of sample values, it is possible to identify a useful signal even under unfavorable circumstances, such that it is possible to cope with even relatively large distances or background scenarios that are noisy or beset by disturbances. In the prior art, sampling is effected by sampling many identical pulses with the time window or phase being shifted, wherein at the present time it is possible to realize very fast circuits having a frequency high enough to sample individual pulses.
The requirements made of the signal sampling and the prerequisites for signal reconstruction are problematic, however, particularly with the use of variable or distorted signals. On account of the sampling rates that are subject technically to upper limits, not all signal components can be sampled in the same way. If the so-called Nyquist sampling theorem is not complied with, then so-called aliasing effects can occur, which corrupt the signal reconstruction and thus reduce the measurement accuracy. The prior art discloses solutions here which nevertheless accept a slight violation of the Nyquist condition or else reduce the higher-frequency signal components by filtering to an extent such that the Nyquist condition can be fulfilled for the filtered signal.
In this regard, WO 2011/076907 discloses a device for highly accurate distance measurement according to the principle of direct sampling of the reflected signal, wherein the reception signal is sampled by a sampling circuit and subsequently quantized. Upstream of the sampling, a high-order filter is allocated to the reception signal path. Said filter is typically a 6th order filter or higher order filter and, unlike in the case of the other devices according to the prior art, is not designed as a simple 1st, 2nd or at most 3rd order antialiasing filter. In the case of such a distance measuring method, a complete identification of the waveform is no longer absolutely necessary. Since, before sampling, the signal bandwidth is reduced such that all frequencies relevant to the distance are below half the sampling frequency, according to the Nyquist theorem the distance-relevant signal that then remains and suffices for the intended measurement purpose can be completely reconstructed by algorithmic means and its exact position can therefore also be determined. The measurement still functions even in the case of varying signals and the accuracy can be increased by this approach. The prerequisite guides a reconstruction is, however, that the dominant portion of the signal must lie within a Nyquist band, the first frequency band preferably being used.
It is true that other methods or devices from the prior art comply with the Nyquist or Shannon condition to a first approximation. In this case, the bandwidth BW or the 3 dB fall-off point f3 dB of the signal spectrum is limited to frequencies below the Nyquist limit frequency fg. However, since the fall-off of the spectrum at frequencies above the BW or f3 dB is without exception too moderate on account of the low filter order, the Shannon theorem is not fulfilled 100% and the suppression of aliasing effects for a precise, offset-free distance measurement is not provided.
The solutions in the prior art thus use sophisticated filter concepts that ensure compliance with the sampling theorem, but they are unable to avoid aliasing effects to the extent required for highly precise measurements.